The present invention relates generally to apparatus responsive to temperature and, more specifically, to an improved pneumatic thermostat to be used in conjunction with air flow distribution systems.
Systems for automatically controlling the environmental temperature in habitable, enclosed areas generally employ a fan pressurized air source which supplies air through ducts to the various rooms or portions of the enclosure. In each room one or more terminal ports may be found which typically employ ceiling-mounted diffusers in order to direct the discharged air horizontally along the ceiling. If pressure increases in the duct system causing the flow through the diffusers to become excessively high objectionable velocity noise may result. On the other hand, if the flow becomes excessively low, the air does not move horizontally along the ceiling when discharged from the diffuser, but rather drops directly down causing what is known as cold air dumping. Such a phenomena may result in objectionable, cold downdrafts on the occupants of the room.
Therefore, in order to provide an air distribution apparatus which maintains the air flow between some acceptable maximum and minimum values, air flow regulators are employed which automatically regulate the volume flow of air from the air supply means to the terminal diffuser ports. Such regulators in order to provide maximum efficiency and optimum service must be responsive not only to pressure differentials within the air distribution apparatus itself, but they must also be responsive to changes in the ambient temperature within the enclosed area. Accordingly, air flow distribution systems typically employ a thermostat disposed within the enclosed area which generates a signal in response to the ambient temperature therein. This signal is transmitted to the volume flow regulator which adjusts the air flow to the terminal port in response thereto. Thus, the air distribution system is self-adjusting so as to maintain the temperature within the enclosure within acceptable limits.
Pneumatic thermostats have been suggested for use in such air distribution systems. These temperature responsive devices generate from a substantially constant pressure air supply a pressurized air signal indicative of ambient temperature. When the system employs a more sophisticated volume flow regulator, it is sometimes desirable that the thermostat be able to reference its output signal not only to temperature but also to the pressure in the system downstream from the regulator. An air distribution apparatus requiring such a thermostat is described in U.S. Pat. application Ser. No. 416,202, now abandoned. However, in order to reference the ouput signal of the thermostat totally to the pressure downstream of the regulator, it is required that the thermometric element be sealed from the ambient air. This, in turn, makes accurate sensing of the ambient temperature more difficult.
Moreover, in order to generate a pressurized air signal which is indicative of ambient temperature the pneumatic thermostat must be provided with a substantially constant pressure air supply. Prior art devices typically employ a central air supply which feeds all the pneumatic thermostats of the air distribution system. Such an arrangement is not only costly, but it also renders the operation of the individual flow regulators dependent upon the central air supply to its thermostat. Thus, the individual air flow regulators are not completely independent.
Therefore, according to the present invention, there is provided a pneumatic thermostat for use in conjunction with an air distribution system which is capable of generating a pressurized air signal referenced not only to ambient temperature but also to the pressure in the distribution system downstream from the air flow regulator. Moreover, the pneumatic thermostat of the present invention uses as its air supply source the air flow within the air distribution system itself. Thus, the need for a central air supply for each individual thermostat is completely eliminated.